In recent years there has been growing interest in OE devices, for example OFETs for use in display devices and logic capable circuits, or organic photovoltaic (OPV) devices.
A typical OFET device includes a substrate, source and drain electrodes that are spaced apart to define a channel region therebetween, a semiconducting layer comprising an organic semiconductor (OSC) material provided at least in the channel region, a gate dielectric layer, and a gate electrode. In addition, the OFET may comprise one or more bank structures, which may for example be patterned such that they define a well extending over the channel region, wherein the OSC layer material can be deposited.
The gate dielectric layer and the OSC layer usually comprise organic materials that are deposited from solution. Therefore the solvents used for coating the gate dielectric layer and the OSC layer, respectively, should be selected from orthogonal systems, to avoid that the layer that is coated first is dissolved by the solvent of the layer that is coated second.
As a consequence, top gate OFET devices typically include a gate dielectric layer comprising a fluorinated material, usually an organic fluoropolymer, because the previously deposited OSC layer is typically selected from materials that are soluble in a broad range of solvents, so that the only solvents for depositing the gate dielectric layer, which are orthogonal to the OSC layer, are fluorinated solvents.
However, the drawback of such gate dielectric layers is the low surface energy of the fluorinated material. The contact angle of water for such layers can be >110°, and it is very difficult to coat or print further device layers onto the surface of such fluorinated gate dielectric layer. In addition, the adhesion of further device layers, like the gate electrode, deposited onto such fluorinated gate dielectric layers is usually very low.
To overcome these drawbacks and enable printing or coating onto a fluorinated dielectric film, plasma treatment or chemical treatment have been proposed for increasing the surface energy by creating new functional groups on the surface. Alternatively, additionally coatings have been proposed to enable printing on top on the dielectric film. However, the adhesion of such films is often poor, and is not compatible with process integration.
Other drawbacks of conventional OE devices, like for example OFETs or OLEDs, are related to de-wetting problems at the bank structures. Thus, if fluorinated bank structure materials are used, other materials, like for example the OSC, that are deposited in the well defined by the bank structures, can de-wet from the walls of the bank structures. Therefore a higher surface energy in these parts of the bank structures is desired. On the other hand, the top surface of the bank structures should have a lower surface energy to avoid wetting with such other materials during their deposition. Therefore, it is desired that bank structures have different surface properties in order to create wetting contrast in a way that other materials de-wet from the bank structures into the defined well therebetween.
Therefore, there is still a need for suitable methods to modify the surface energy of fluorinated insulating structures, like gate dielectric layers, in OE devices like OFETs, especially in top gate OFETs, which fulfil the requirements and overcome the drawbacks as discussed above. In particular, there is a need for a method to increase the surface energy of insulating structures in OE devices, in particular in bank structures or gate dielectric layers, and to increase the adhesion of gate dielectric layers to further layers deposited thereon.
One aim of the present invention is to provide such improved methods. Another aim is to provide improved OE devices prepared by such methods. Further aims are immediately evident to the person skilled in the art from the following description.
The inventors have found these aims can be achieved by providing a method of modifying the surface energy of fluorinated insulating structures in accordance with the present invention and as claimed hereinafter.